1. Field of the Invention
This invention relates to the detection of flaws in conductive objects, and more particularly, an improved eddy current flaw detector which isolates the precise location of flaws more accurately than prior art detectors.
2. Background Information
Non-destructive testing is used to test objects for flaws without damage to the object. In the case of the present invention, non-destructive testing of conductive objects is accomplished by inspecting for discontinuities by taking advantage of the object's ferro-magnetic and electrical conductivity properties.
The term "conductive object" is not intended to limit the class of testable objects to those typically used as electrical conductors, such as copper or aluminum. Rather, it is contemplated that the present invention could be used to test objects with limited electrical conductivity such as graphite.
When a coil of wire, through which alternating current is flowing, is placed in the vicinity of an object which is a good electrical conductor or has a high magnetic permeability, the impedance of the coil will change. If the coil is kept a fixed distance from the object and moved adjacent to the surface of the object the impedance of the coil will remain generally constant if the object is continuous. However, if the coil is moved in the vicinity of a discontinuity, such as a flaw, the coil's impedance will change from the value it assumed when out of the flaw's vicinity. It is that change in impedance which the present invention takes advantage of in operation.
A number of devices have been proposed for testing for flaws in conductive objects.
U.S. Pat. No. 4,507,610 discloses an apparatus for electromagnetically detecting flaws in metal objects which includes a coil of wire in which is disposed a split ring which is made of an electrically conductive material. Attached to the ends of the split ring is a sensing head which is used to couple the coil of wire to the metal object being tested. In use, the coil is excited with a high frequency electrical current which induces an electrical current in the closed circuit formed by the split ring and sensing head. When the sensing head is moved in the vicinity of a flaw, the impedance of the coil changes, thereby indicating the presence of a flaw. This device has the serious drawback of poor resolution due to the size and shape of the sensing head.
In addition to that device, a commonly used eddy current device, for the detection of flaws in nuclear plant steam generator tubing, is two tandem coils to provide a differential signal between the two coils. That apparatus, however, fails to provide distinct resolution which precisely locates the flaw in the metal, as it fails to provide angular resolution around tubular objects which are being tested. The employment of cross-wound coils is one variation of this device, however, they provide resolution sufficient to locate a flaw in only one hemicylinder of the sensor or the other.
Further, it has been known to shape the core of magnetic coils, to provide varying resolution patterns, when the core is to be used as the probe or sensor for the detection of flaws. For example, pointed ends have been formed on cores to provide more precise resolution. Also, other versions have horseshoe shaped cores which focus the flux at a desired point.
Also, it has been known to use two coils, a source coil and a detector coil, to detect flaws in conductive objects. With these devices, the source coil induces an electrical current in the detector coil and the detector coil's current is then monitored. When the detector coil is disposed in the vicinity of a flaw its impedance changes, thereby signalling the presence of a flaw. In some applications both coils are disposed on one side of the object while other applications require that the coils be placed on opposite surfaces of the object. The use of two coils requires more space, than those detectors which employ only one coil, and provides poor angular resolution. Also, in those applications requiring the coils to be disposed on opposite surfaces of the object, accessibility to both surfaces may be limited, restricting the amount of surface area which may be tested.
Therefore, despite the above-described devices there exists a real need for an improved electromagnetic sensor which is capable of pinpointing the location of flaws in conductive objects.
It is a primary object of the present invention to provide a device which is capable of accurately nondestructively locating flaws in conductive objects.
It is another object of the invention to produce a sensor which may be conveniently moved inside and along the longitudinal length of steam generator tubing in a nuclear steam supply system.
It is a further object of the invention to provide a sensor which is economical to manufacture and simple to operate.
It is yet another object of the present invention to provide a sensor which does not require the use of an external magnetic field to operate.